Technical Field
This disclosure relates in general to redox flow battery (RFB) systems for energy storage, and in particular to the so-called all-vanadium RFB system. This disclosure addresses the problem of monitoring the state of charge of the positive electrolyte solution and of the negative electrolyte solution.
Related Art
RFB energy storage systems [1-7] are recognized as particularly efficient and flexible candidates for large scale energy storage requirements of intelligent power distribution networks being developed.
The all-vanadium (V/V) RFB system using the redox couples V+2/V+3 in the negative electrolyte solution and V+4/V+5 in the positive electrolyte solution is probably the one that has had significant industrial applications and that is most extensively studied. Other similar RFB systems like Fe/V, V/Br, Cr/Fe, Zn/Ce, Polysulfide/Br, have been studied but have not had a comparable commercial acceptance. A common feature to these systems is that, for economically acceptable current densities to be supported, porous and fluid permeable electrodes are necessary. Moreover, chemical inertness of the electrode materials that need to be retained when switching from cathodic polarization to anodic polarization during a cycle of charging and discharging of the redox storage system, and the requisite of having a relatively high H+ discharge over-voltage when negatively polarized in respect to the electrolyte solution and a high Off discharge over-voltage when positively polarized in respect to the electrolyte solution, obliges to use carbon base electrodes.
Yet, preventing parasitic OH− discharge and/or H+ discharge in case of localized depletion of oxidable and reducible vanadium ions of the respective redox couples in the two solutions because of non uniform mass transport and/or electrical potential throughout porous electrode felts of non woven activated carbon fibers, generally sandwiched between the ion permeable cell separating membrane and the surface of a conductive current distributing plate, remains a critical aspect.
Parasitic oxygen discharge at the carbon electrode may accidentally becomes the main current supporting anodic reaction if the design maximum current density limit is for some reasons surpassed or if the charging process is accidentally protracted beyond full vanadium oxidation in a positive electrolyte solution to V+5. In the latter event, another serious effect may start to manifest itself, notably a gradual precipitation of vanadium pentoxide according to the reaction: 2VO2++H2O=V2O5+2H+.
The first of these hazardous occurrences may lead to a rapid destruction of the carbon felt and of the carbon-based current collecting plates by nascent oxygen with generation of CO and CO2. For this reason many substances have been identified as poisoning agents of oxygen evolution on carbon anodes in the typical sulphuric acid electrolyte solutions of vanadium RFBs like antimony (Sb+3), Borax and tellurium (Te+4), generally preferred because besides raising the oxygen evolution over-voltage, they also poisons H+ discharge in case of migration/contamination of the negative electrolyte solution. The second occurrence, if unchecked, causes clogging most likely in the pores of the carbon felt electrode, which is particularly difficult to remedy, and unbalancing of the electrolytes. As it is well known, parasitic hydrogen evolution in a vanadium RFB energy storage cell may be favoured by accidental contamination of the electrolyte solutions with metals having a low hydrogen over-voltage like Fe, Ni, Co, . . . etc. that may deposit on the carbon electrode structure, and/or when V+3 has been completely reduced to V+2 in which case the only electrode reaction that may support circulation of electric current becomes the electrolysis of water.
Specific monitoring of working conditions in the cells is indispensable and its shortcomings has been the cause of costly failures. More sophisticated and reliable ways of controlling the operation of RFB energy storage systems are been developed.
Prior patent application No. PCT/IB2012/057342, of the same applicants, discloses a reliable monitoring system of the operation conditions that provides a long sought detectability at single cell level, impossible with the multi-cell bipolar stacks typical of known industrial all-vanadium flow redox batteries. The content of his prior patent application is herein incorporate by express reference.
The technique of monitoring the state of charge of the electrolyte solutions by measuring the open cell voltage (OCV) in a minuscule cell replica of the battery cells through which diverted streams of the positive and negative solutions flow as depicted in FIG. 1, or in a simplified though equivalent manner described in said prior application, is well known. However, what is measured is the “overall” state of charge and any intervened unbalance between the state of charge of the negative and positive electrolyte solutions remains undetected. Given that in all-vanadium RFB systems and mutatis-mutandis also in other RFB systems a perfect symmetrical reduction and oxidation of the redox ion couples respectively used in the negative and in the positive electrolyte solutions can hardly be retained over many charge/discharge cycles, the risk remains of running into critical limit conditions in one or the other of the two electrolyte solutions.
As widely accepted, in all-vanadium RFB systems the causes of unbalance are oxidation of reduced vanadium ions V+2 by contact with ambient air in the tank and parasitic hydrogen evolution (gassing) occurring on the negative electrode. This progressively leads to a state of charge of the positive electrolyte solution exceeding the state of charge of the negative electrolyte solution. The opposite condition of unbalance cannot occur in practice.
An accumulated unbalance of charge between the two electrolyte solutions, the effect of which being that a measured OCV of magnitude short of the one expected at full charge may mask the fact that the positive electrolyte solution has reached a condition of full charge (all vanadium oxidized to V+5) whilst the negative electrolyte solution has not yet reached a complete reduction of all vanadium to V+2, but just a partial reduction in a V+2.4−V+2.6 range. This normally occurs when periodically re-mixing the two electrolyte solutions for re-establishing a volumetric and/or constituents balance of the two solutions, as it is generally practiced (easier than adjustments by other ways). This mechanism, besides progressively reducing the storage capacity really available, poses serious risks of damaging the positive carbon felt electrodes of the cells because of a concurrent/substitute oxygen discharge through electrolysis of the water solvent.
There is an evident need of monitoring the state of charge of the single electrolyte solution that in the case of an all-vanadium RFB system point to the positive electrolyte solution as the critical one to be monitored. This requires the use of standard reference electrodes. Proposed alternatives to the use of expensive and bulky instruments like a standard hydrogen electrode, have not sorted satisfactory results in terms of precision and reliability.